Li3Cs2M2B3P6O24 (M = Pb, Sr): borophosphates with double six-membered ring of [BP2O8]3−.

New borophosphates Li₃Cs₂M₂B₃P₆O₂₄ (M = Pb, Sr) have been synthesized by high-temperature solid-state reactions. The single-crystal X-ray diffraction data reveal their non-centrosymmetric cubic space group P2₁3 and lattice parameters of a = 12.7246(3) Å and 12.687(2) Å and V = 2060.31(8) ų and 2042.3(4) ų for Li₃Cs₂Pb₂B₃P₆O₂₄ and Li₃Cs₂Sr₂B₃P₆O₂₄, respectively. They are three-dimensional (3D) [BP₂O₈]³⁻_∞ anionic frameworks formed by a new double six-membered ring building unit consisting of BO₄ and PO₄ units joined alternately. Interestingly, such a double six-membered ring is regarded as the structural variation of the 12-membered ring in the known KSrBP₂O₈, fusing at the BO₄ tetrahedron induced by the coexistence of the Li⁺ and Cs⁺ cations with their large size difference. The title compounds show weak SHG intensity and exhibit photoluminescence emissions at 410 nm for Li₃Cs₂Pb₂B₃P₆O₂₄ and 440 nm for Li₃Cs₂Sr₂B₃P₆O₂₄. [ABSTRACT FROM AUTHOR]